1. Field of the Invention
The present invention relates to mobile communication systems and their control methods and, more particularly, to a mobile communication system and a control method that avoids radio interference and crosstalk to message or speech channels avoids crossfire or disturbance to a control channels and enables the selection of the speech channels.
2. Description of the Related Art
There has been suggested a mobile communication system in which a plurality of fixed radio transceivers are positioned so that their radio zones (or service areas) partially overlap with each other, whereby communication can be achieved between mobile radio transceivers moving within radio zones, of the fixed tranceivers and wired telephone lines.
FIG. 1 shows a block diagram showing an arrangement of such a sort of mobile communication system. The system of FIG. 1 comprises mobile radio transceivers 3a to 3e, fixed radio transceivers 41 to 43 and a line controller 7. The fixed radio transceivers 41 to 43 have radio zones 51 to 53 respectively. Connected to the line controller 7 are wired telephone lines 6a to 6e which correspond to the mobile radio transceivers 3a to 3e respectively.
The line connecting operation of the mobile communication system will now be explained. Assume that there is an incoming call on the wired telephone line 6a. The line controller 7 then activates the fixed transceiver 41 of the radio zone 51 in which the mobile transceiver 3a corresponding to the wired telephone line 6a is present, and transmits a call indication signal to the mobile transceiver 3a. When the transceiver 3a receives the call indicator signal it, rings its sounder to inform the user of the transceiver 3a of the incoming of a call. When the user responds by picking up the telephone receiver to put the telephone in its off-hook mode, the mobile transceiver 3a sends a call answering signal back to the line controller 7 via the fixed radio transceiver 41. The line controller 7, when detecting the answering signal, connects the wired telephone line 6a to the fixed transceiver 41 to establish speech communication between the party at the other end of the line 6a and the user of the mobile transceiver 3a.
When the user transmits an outgoing call from the mobile transceiver 3a through the fixed transceiver 41 to the line controller 7, the line controller 7 acts to interconnect the wired telephone line 6a and the fixed transceiver 41 and call the desired party leading to the wired telephone network.
In the aforementioned calling and called operations, the speech channel established between the fixed and mobile transceivers 41 and 3a corresponds to an idle speech channel which the controller has retrieved among a plurality of speech channels when the fixed transceiver 41 is in its await mode. When the fixed transceiver sends information on the idle speech channel to the mobile transceiver 3a during calling operation of the mobile transceiver 3a, this causes the fixed and mobile transceivers 41 and 3a to be both switched to the idle speech channel, thus realizing a speech therebetween.
The retrieval time of the idle channel for the fixed and mobile transceivers 41 to 43 and 3a to 3e is restricted to be a time as small as below 100 ms to minimize a response time to a call or a received call. For this reason, this system has had such a problem that, when the electric field intensity of another speech channel already in service at the retrieval timing of the idle channel is reduced due to fading or the like, the fixed transceivers 41 to 43, or the mobile transceivers 3a to 3e, may erroneously judge an in-use speech channel to be idle, thus causing interference between the speech channels. In addition, the system is also defective in that the movement of the mobile transceiver during its speech operation causes the change of propagation state of the electromagnetic waves, thus resulting in occurrence of interference on an identical speech channel.
To eliminate such identical-speech-channel interference, there has been proposed such a method that an interference detecting circuit is provided for each of the fixed and mobile transceivers to switch the current speech channel to a one related to a frequency causing no interference. However, in the case where a frequency difference between the both channels is small, it is difficult to detect interference itself and thus it is impossible to reliably prevent interference.
Furthermore, if the frequencies of two fixed radio transceivers A and B satisfy the following relation, EQU fc=2.multidot.(fc+.DELTA.fs)-(fc+2.DELTA.fs)
where fc is the frequency of the control channel, .DELTA.fs is an interval between speech channels, fc+.DELTA.fs is the frequency of transmit waves of the fixed transceiver A, and fc+2.DELTA.fs is the frequency of transmit waves of the fixed transceiver B, then there has occurred such a problem that radio waves of the same frequency as the control channel are generated due to intermodulation between the respective transmit waves of the both fixed transceivers A and B, thus causing the crosfire or disturbance of the control channel.
It is an object of the present invention to a mobile communication system and a control method that can avoid these problems of the prior art, avoid radio interference and crosstalk to message or speech channels, avoid crossfire or disturbance to a control channel, and realize stable speech communiction.